Douglas Aircraft began initial design work for what would ultimately become the DC-9 in 1962. The aircraft was especially designed to operate effectively at smaller regional airports with smaller terminals and shorter runways, explaining its narrow body, relatively small wings, and rear mounted engines.

The plane was always intended to have stretched variants that could carry more passengers; in addition to the initial model, designated the DC-9-10 series (with the -15 variant being the most common), the -20, -30, -40, and -50 stretched variants were rapidly introduced. Three final variant series were produced under different names, with the -80 and -90 being renamed to the McDonnell Douglas MD-80 and the McDonnell Douglas MD-90 to make them sound like newer airplanes, and the -95 variant being renamed the Boeing 717 after Boeing bought McDonnell Douglas in 1997.